<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      var a = 1;

      function fn1() {
        function fn2() {
          console.log(a);
        }
        function fn3() {
          var a = 4;
          fn2();
        }
        var a = 2;
        return fn3;
      }

      var fn = fn1();
      fn(); //输出2
    </script>
  </body>
</html>
